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Línekite, K2Ca3[(UO2)(CO3)3]2.8H2O, is a new uranyl tricarbonate mineral from Jáchymov, Western Bohemia, Czech Republic. It occurs in association with grimselite, andersonite, liebigite, čejkaite, schröckingerite, agricolaite, ježekite and braunerite. Línekite forms from uranium-rich aqueous solutions and its origin is associated with post-mining processes. Línekite is orthorhombic, space group Pnnm, with a = 17.0069(5) Å, b = 18.0273(5) Å, c = 18.3374(5) Å and V = 5622.1(2) Å3, and Z = 8. It forms tabular, mostly isometric crystals, up to c. 0.5 mm across, typically in multiple intergrowths. The color is pale olive to khaki green and it has a greenish white to yellowish white streak. Crystals are transparent and have vitreous luster. The Mohs hardness is estimated to be between 2 and 3. Línekite is brittle with an uneven fracture and perfect cleavage on {100} and very good cleavage on {010}. It exhibits intense greenish yellow luminescence under both short- (254 nm) and long-wave (366 nm) UV radiation. The calculated density is 2.922 g/cm3. The mineral is biaxial (+) with indices of refraction, α = 1.546(2), β = 1.550, γ = 1.562(2). The 2Vobs is moderate; the calculated 2V is +60°. Optical orientation: Y = a, X = b, Z = c. The electron microprobe analyses (average of 28) provided: Na2O 0.06, K2O 6.89, CaO 14.11, CuO 0.12, UO3 48.76, CO2* 22.51, H2O* 12.20 (~12.9 from TG) (*calculated), total 104.65 wt%. The empirical formula (based on 30.22 O apfu) is: (K1.73Na0.02)Ʃ1.75(Ca2.97Cu0.02)Ʃ2.99[(UO2)(CO3)3]2.02(H2O)8.00. The Raman and infrared spectra exhibit prominent features consistent with the mineral being a hydrated uranyl tricarbonate, with fundamental vibrations of H2O molecules, CO3 2- anions and UO2 2+ ions. The seven strongest powder X-ray diffraction lines are [dobs in Å (hkl) Irel]: 8.627 (200) 100, 6.436 (022) 60, 5.935 (212) 11, 5.153 (222) 43, 4.592 (004) 19, 4.505 (040) 12 and 4.053 (204) 15. The structure of línekite was refined from single-crystal X-ray data to R = 0.034 for 4468 unique observed reflections (Iobs > 30I). The structure consists of prominent (Ca(H2O)2[(UO2)(CO3)3])2- layers parallel to (100), which define a square grid, leading to a strong tetragonal pseudosymmetry of línekite. Between the layers, disordered K+ cations and H2O molecules are localized. The structure is closely related to other uranyl tricarbonate minerals, e.g., albrechtschraufite and andersonite, due to the presence of a very characteristic paddle-wheel motif, Ca[(UO2)(CO3)3]4Ca. [ABSTRACT FROM AUTHOR]